Flow cells are typically used in optical detectors for measuring the optical characteristics of a sample liquid flowing through the flow cell. Such detectors with the corresponding flow cell are used, for example, in liquid chromatography, or in capillary electrochromatography, or capillary electrophoresis, for determining the identity and concentration of sample substances. Ultraviolet (UV) and visible (Vis) light detectors wherein the absorbance of a sample in the flow cell is measured are frequently used for the mentioned separation techniques. Known flow cells are typically made of steel or quartz and have a cylindrical bore through which the liquid flows and through which light for analyzing the sample is directed.
It is important that a detector has: a) a high measuring sensitivity in order to be able to detect small sample amounts, and b) a small flow cell volume in order to ensure satisfactory peak resolution. Unfortunately, these are conflicting requirements since in the mentioned flow cells an increase of the sensitivity at constant cell volume leads to a reduction of the light conductivity value which in turn increases the noise and thus limits the detection limit. A solution to this problem consists in the use of flow cells employing total internal reflection of the light transmitted through the flow cell. This is achieved by coating the internal walls of the flow cell with a polymer having an index of refraction below that of water. A suitable polymer for this purpose is an amorphous fluoropolymer such as Teflon® AF (trademark of Dupont ). Due to the loss-free reflection in these cells, the light conductivity value is not reduced, so that the pathlength of the cell can be increased without loss in light throughput. Flow cells using the principle of total internal reflection are known from U.S. Pat. No. 5,184,192, U.S. Pat. No. 5,444,807, U.S. Pat. No. 5,608,517, and U.S. Pat. No. 6,188,813.
The mentioned patents also describe different methods for manufacturing the flow cells. According to U.S. Pat. No. 5,608,517, a mandrel is used which is coated with Teflon AF by multiple dipping into a corresponding solution and subsequent curing. The coated mandrel is then inserted into a housing comprising a heat shrinkable tubing. This tubing comprises an inner melting layer. The assembly is then heated to shrink the outer layer and to melt the inner layer. The melting inner layer fuses to the Teflon AF layer on the mandrel upon cooling. The mandrel is then removed from the flow passage leaving the Teflon AF layer fused and bonded to the inner layer. A shrinking step has disadvantages, because any inhomogeneities in the shrinking material lead to bending of the Teflon AF tube which causes a deterioration of the light throughput. In addition, the known process is complicated and costly because measures have to be taken to prevent damage of the Teflon AF layer when removing the mandrel and to ensure that the Teflon layer remains stable after removal of the mandrel.
According to U.S. Pat. No. 6,188,813, a molding step is employed. This, however, has the disadvantage that due to the direction of inflow and the mass of the molding material the soft Teflon AF tube may be deformed during the molding step, particularly with long cells. A consequence is that the optical properties, in particular the total reflection properties, are impaired. If there is a local unevenness in the Teflon AF material, the reflection angles will increase due to increased scattering so that the critical angle for total reflection is exceeded. The result is the loss of light and thus unwanted influences on the signal, for example an increase in the sensitivity to refractive index changes.